This invention relates to an improved method of molding and producing fiber reinforced articles. In particular, the invention relates to a method for the high-speed production of articles from glass fiber reinforced plastics and polymers. The present invention facilitates the high volume production of large structural parts such as automotive primary structures.
A commonly accepted practice for the process of high-speed molding is disclosed in U.S. Pat. No. 3,809,739. This process involves the use of a plurality of latching mold dies which receive a charge of molding material, such as a sheet or mat, uncured molding compound (normally a resin material) and glass fiber reinforcement. The mat or sheet is formed around a lower die member and an upper die member is then securely latched to the lower die member. The molding pressure of the closed die members causes the sheet molding compound to flow over the mold, sealing the sheet or mat into the mold cavity of the mold die. The loaded mold die is pressed in a mold press, cured and the molded article is then removed. However, problems have been encountered with this process when attempting the high speed production of large structural items. If the mold dies have sharp angles and deep contours into which the reinforcement matting must be pressed, there is a tendency for the mat to flow unevenly while being pressed into the mold.
In addition, the present processes are also hampered by the size limitations of the mats. Fiber reinforced products having a large size are currently produced by placing two smaller mats adjacent each other in a mold. Pressure is applied to the mats such that the edges of the adjacent mats flow toward one another under sufficient pressure to come into contact with each other. The smaller mats are thus formed into the larger mat having what is known as a "knitline" where the edges of the mats are joined together. However, as pressure is applied to the mats the resin molding compound flows more easily than the glass fiber reinforcement. The result is that there may be gaps or weak spots in the mat where relatively little or uneven amounts of glass fiber reinforcement are present along the knitline.
Further, the present molding processes encounter such problems as where the glass fiber strands of the mat separate or tear at the deeply contoured areas or sharp edges due to the pressures of the molding process. This results in an uneven distribution of the reinforcement mat during the molding process and produces a final molded product having inadequate structural strength in crucial areas.
Past efforts to improve upon this basic process have added a method step of preprocessing the mat or sheet into a "preform" which generally assumes the shape of the final molded product. The preform process generally includes the steps of chopping the glass fiber and feeding the glass fiber to a preform screen while applying a binder material. The composite glass/binder combination is then cured into a preform. The preform is then removed from the preform screen. The preform can then be stored or shipped for use in a later molding process to make a final fiber reinforced product. The additional step of making a preform assures greater distribution of the glass fiber and provides enhanced structural integrity of the final molded article. However, the added preforming step has been found to be undesirable in some manufacturing processes due to the necessary increase in manufacturing time. The use of chopped glass fiber may also, in some instances, continue to cause the preform to suffer similar problems of separation and tear out in crucial structural areas due to the pressures applied during the molding step.
Additional problems are encountered in the use of preforms of chopped glass fiber when the resin is injected into the screen. In particular, the placement of the chopped glass fiber on the screen is often imprecise, causing gaps in the glass fiber reinforcement. Also, the pressure applied by the screen to the chopped glass fiber and resin cause heavy areas of concentration of the chopped glass and thin sections of the resin. This uneveness causes the glass fiber to move about on the screen. In addition, the chopped glass has a tendency to clump together or be compacted into a very dense area such that when the resin is applied to the chopped glass the resin may be washed off rather than permeate the chopped glass.
The molding system of the present invention improves upon the quality and speed with which such large structural fiber reinforced structures can be produced at high volume.
Accordingly, an object of this invention is to reduce the number of manufacturing steps in the molding process of large structural fiber reinforced products.
Another object of this invention is to increase the quality of the distribution of glass fibers throughout the fiber reinforced product.
Yet another object of this invention is to provide a molded product in which the resin is easily injected through the glass fiber mat to thoroughly permeate the glass fiber preform.
These objects are achieved with the present invention by directly distributing continuous strand glass fiber to a preform screen from a glass fiber manufacturing apparatus. A binder material is applied as multiple layers of the continuous strand glass fiber are placed upon the preform screen. The binder material can be applied as a liquid material, a slurry, or a dry particulate material. The distribution of the glass fiber strands achieves the desirable fiber distribution and structural strength of the preform. The preform can be used immediately in a molding process or can be stored or shipped for a later use.
In the molding process the preform is cured and then is placed into a mold. The preform is sealed in the mold, injected with a resin material and allowed to cure. After the molded preform article is cured, it is removed from the mold. The present invention successfully achieves the high speed production of large structural parts. The present invention eliminates the necessity of chopping the glass fiber for feeding to a preform screen. The present invention also eliminates the need to apply pressure to the mold containing the preform during the molding process.
Other objects and advantages of the invention will be apparent from the following description of the preferred embodiment thereof, with reference being made to the accompanying drawings.